On the first 3 quizzes of her math class, Emily got an average score of 78. What does she need on the next quiz to have an overall average of 80?
Answer: Let her score on the next quiz be $x$ The sum of all of her scores is then $3 \cdot 78 + x$ The same sum must also be equal to $4 \cdot 80$ Solve: $x = 4 \cdot 80 - 3 \cdot 78 = 86$.